Power supply protection circuit

ABSTRACT

A circuit is disclosed for protecting a power supply including a Royer type inverter circuit from damage due to overloading. The protective circuit includes a capacitor for triggering a normally non-conductive silicon controlled rectifier in response to an increase in voltage across the switching transistors of the Royer circuit caused by overloading. When triggered, the silicon controlled rectifier reduces the output of a Darlington amplifier, which in turn reduces the power delivered to the switching transistors. The protective circuit may include a network for automatically returning the silicon controlled rectifier to its non-conductive state, thereby restoring normal operation of the overall power supply circuit.

United States Patent Fendrich, Jr. 1 1 Jan. 30, 1973 [54] POWER SUPPLYPROTECTION CIRCUIT v '7 Primary Examiner]a mes D. Trammell [75]Inventor: Charles Nelson Fendrich, Jr., Attorney-William Keatmg atEhzabethtown, Pa. [57] ABSTRACT [73] Asslgnee: AMP Incorporated HamsburgA circuit is disclosed for protecting a power supply in- [22] Filed:April 10, 1972 eluding a Royer type inverter circuit from damage due tooverloading. The protective circuit includes a [21] App! 242541capacitor for triggering a normally non-conductive silicon controlledrectifier in response to an increase in 521 vs. e1 ..317/22, 317/31,317/33 sc, voltage across the Switching transistors of the R Y" 317/49321/14 circuit caused by overloading. When triggered, the sil- 511 Int.Cl. .ittizh 7/10 mmmlled rectifier reduces the a [58] Field of Search317/22 33 SC 31 321/14 Darlington amplifier, which in turn reduces thepower delivered to the switching transistors. The protective circuit mayinclude a network for automatically [56] References Clted returning thesilicon controlled rectifier to its non- UNITED STATES PATENTSconductive state, thereby restoring normal operation of the overallpower supply circuit. 3,386,005 5/1968 Roland et al. ..3l7/22 3,531,7119/1970 Fuseo ..321/I4 10 Claims, 1 Drawing Figure l i INVERTER I cmcun lPRIMARY 1 INVERTER I CIRCUIT I 2/ sEcomARY I Q l 1 I DARLINGTONAMPLIFIER I I42 I I 11 I no. I "W SOURCE l 76 I l l l J POWER SUPPLYPROTECTION CIRCUIT BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates generally to protective circuits, and moreparticularly to a silicon controlled rectifier circuit for protectingpower supplies from damage due to overloading.

2. Description of the Prior Art It is well known that solid state powersupplies require protection against overload conditions, since overloadscan cause excessive power dissipation and subsequent overheating anddestruction of solid state components. The use of conventional fuses isan inadequate solution to this problem, since fuses are relatively slowacting devices, and cannot conveniently be automatically reset after theoverload condition has passed.

Accordingly, fast acting silicon controlled rectifier circuits have beenproposed in the past. For example, a high-speed, self-restoring solidstate protective circuit which includes a silicon controlled rectifieris disclosed in US. Pat. No. 3,386,005 to Roland et al., issuedMay 28,1968. The protective circuit described in this reference includes manyhighly desirable and advantageous features. However, this circuit has atleast one significant disadvantage in that the silicon controlledrectifier included in it is triggered by excessive load current. Sincethe load may be a cathode ray tube for example, the normal load currentis relatively low, ranging from approximately 200 microamps to only afew milliamps under normal conditions. Thus, a large triggering resistormust be included in the gate circuit of the silicon controlled rectifierdisclosed in Roland et al. in order to develop a sufficient triggeringvoltage. However, the use of a large resistor in the gate circuit of thesilicon controlled rectifier results in low switching sensitivity, andcan cause the circuit to be sensitive to spurious AC signals. Inaddition, the protective circuit disclosed in the Roland et al.,reference must dissipate a significant amount of power to sense overloadcurrents, and is therefore somewhat inefficient in its operation.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is toprovide an improved overload protection circuit which is relativelyinsensitive to spurious AC signals.

Another object of this invention is' to provide an overload protectioncircuit which dissipates minimum power in detecting overload conditions.

Yet another object of this invention is to provide a novel protectivecircuit which detects overloading of the-driving transistors in a powersupply.

A still further object of this invention is to provide a novel overloadprotection circuit which is voltage rather than current sensitive.

A still further object of this invention is to provide a solid stateoverload protection circuit of improved switching sensitivity andreliability.

Briefly, these and other objects of the invention are achieved byproviding a silicon controlled rectifier protective circuit which isdirectly responsive to voltage changes across the driving transistors ofa power supply including a Royer type switching circuit. The gateelectrode of the silicon controlled rectifier is coupled through avoltage divider to a capacitor which charges to approximately thevoltage across the driving transistors of the power supply. When thisvoltage increases beyond a predetermined level, the voltage on thecapacitor acts through the voltage divider to trigger the siliconcontrolled rectifier, thereby cutting off the power supply.

BRIEF DESCRIPTION OF THE DRAWING A more complete appreciation of theinvention and many of the attendant advantages thereof will be readilyobtained as the same becomes better understood by reference to thefollowing detailed description when' considered in connection with theaccompanying drawing, wherein:

The FIGURE is a combined circuit and block diagram of the preferredembodiment of the protective circuit of the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT er circuit 16. A differentialamplifier 18 and aninverter primary circuit 20 are driven by the outputof the Darlington amplifier 16. Power is delivered to the load 12through an inverter secondary circuit 22 which is magnetically coupledto the inverter primary circuit 20.

The differential amplifier 18 is preferably of the type illustrated inthe above mentioned Roland et al., patent, although many types ofconventional differential amplifier circuits can be used with thecircuit of the present invention. The differential amplifier 18 comparesthe output of the inverter secondary circuit 22 with a suitablereference potential to regulate the output of the power supply circuit10.

The inverter primary circuit 20 includes a Royer type switching circuitfor converting a direct current input to an alternating current output.The inverter primary circuit 20 is structurally and functionally similarto that disclosed in the above-identified Roland et al., patent,although it has been modified in various respects to accomodate theprotective circuit of the instant invention. Similarly, the invertersecondary circuit 22 is preferably of the type described in theabove-identified Roland et al., patent, although other types ofconventional inverter secondary circuits will operate satisfactorilywith the circuit of the present invention.

A protective circuit 24 is shown coupled between the Darlingtonamplifier 16 and the inverter circuit primary 20. The protective circuit24 includes a silicon controlled rectifier (SCR) 26, the cathode ofwhich is coupled to a ground or other suitable reference potential 28,and the anode of which is coupled to one terminal of acurrent limitingresistor 30. The current limiting resistor 30 is selected to limit themaximum current flow through the SCR 26 to a value less than the maximumcurrent rating of the SCR. The remaining terminal of the currentlimiting resistor 30 is coupled to a base lead 32 in the Darlingtonamplifier 16. The base lead 32 is the control terminal for a pair oftransistors 34 and 36 coupled together in a conventional Darlingtonconfiguration. The transistor 36 is known as the pass transistor of theDarlington pair. A control resistor 38 is coupled between an inputelectrode 40 of the Darlington pair and the base lead 32. A capacitor 42may optionally be coupled between the base lead 32 of the Darlingtonpair and the ground or reference potential 28.

Referring again to the protective circuit 24, the gate electrode of SCR26 is coupled to a voltage divider comprised of a pair of resistors 44and 46. A filter capacitor 48 is also preferably coupled between thegate electrode of SCR 26 and ground 28, in order to filter out voltagespikes or other spurious voltages which might otherwise accidentiallytrigger SCR 26.

The gate electrode of SCR 26 is coupled through voltage dividingresistor 46 to the inverter primary circuit 20. The inverter primarycircuit includes a pair of base driving coils 50 and 52 which arecoupled through a pair of resistors 54 and 56 to the base electrodes ofa pair of output or driving transistors 58 and 60, respectively. Theoutput transistors 58 and 60 drive a center tapped primary coil 62, thecenter tap of which is coupled through a lead 64 to the Darlingtonamplifier 16. The center tap of the primary coil 62 is also coupledthrough a resistor 66 to the center tap or junction between the basedriving coils 50 and 52. The base driving coils 50 and 52 and theprimary coil 62 are preferably mounted on the same saturable core. Thecollectors of the driving transistors 58 and 60 are connected across theprimary coil 62, while the emitters of the two driving transistors areboth coupled to a suitable reference potential such as ground 28. Adiode 68 may also be coupled between the ground 28 and the junctionbetween base driving coils 50and 52. A pair of blocking diodes 70 and 72are coupled to the collectors of driving transistors 58 and 60 at theircathodes, respectively, and are coupled together at their anodes. Aresistor 74 is coupled between the lead 64 and the interconnected anodesof blocking diodes 70 and 72, while a triggering capacitor 76 is coupledbetween ground 28 and the junction of resistor 74 and the interconnectedanodes of blocking diodes 70 and 72. A lead 78 couples the triggeringcapacitor 76 and the gate electrode of SCR 26 through the voltagedividing resistor 46.

When the power supply circuit is operating in its normal mode, power issupplied from the DC source 14 through the Darlington amplifier 16 tothe inverter primary circuit 20. In the inverter primary circuit theoutput or driving transistors function in cooperation with the basedriving coils 50 and 52 and the primary coil 62 as a conventional Royerswitching circuit. That is, driving transistors 58 and 60 which areoppositely phased, alternatively switch from a fully conductive orsaturated state to a completely nonconductive state, thereby developingan alternating current in the primary coil 62, which is picked up by theinverter secondary circuit and subsequently fed to the load 12.

When the power supply circuit 10 is operating in its normal mode, thevoltage on the triggering capacitor 76, which is approximately equal tothe saturation voltage drop across one of the driving transistors 58 and60 plus the voltage drop across either one of the blocking diodes or 72,is quite low. For example, this voltage may be on the order of 1 volt.The resistors 44 and 46 of the voltage divider coupled to the gate ofSCR 26 are chosen so that this voltage which normally exists on thetriggering-capacitor 76 is insufficient to trigger the SCR 26, and theSCR is thus non-conductive when the power supply circuit 10 is operatingnormally. The resistor 46 may have a relatively small value, on theorder of 220 ohms for example, while the resistor 44 may have a somewhatlarger value, on the order of lKohm for example. These relatively lowresistance values in the gate circuit of SCR 26 permit the triggeringcircuit of the SCR to be made very sensitive to overload conditionsexisting in the power supply circuit 10, but relatively insensitive tospurious voltage spikes or AC signals existing in the environment of thepower supply circuit.

As explained above, the voltage which is normally present on thetriggering capacitor 76 is insufficient to fire the SCR 26, and thus theSCR remains in a nonconductive state when the power supply circuit 10 isoperatingin its normal mode. However, when an overload condition arises,the driving transistors 58 and 60 will either begin to come out ofsaturation, due to an inability to supply the excessive currents drawnby the load, or will go into a high frequency oscillation. In eithercase, the voltage on triggering capacitor 76, will increase, deliveringa sufficient potential to the voltage dividing resistors 44 and 46 tofire SCR 26. When the SCR 26 is fired, the potential at the base lead 32of Darlington amplifier 16 is substantially reduced, thereby greatlyreducing the output of the Darlington amplifier. The reduced output ofthe Darlington amplifier effectively shuts off the inverter circuitprimary 20,

thereby preventing damage to the driving transistors 58 and 60, or toany of the other circuit components, due to the overload condition.

If it is desirable that the protective circuit 24 should beself-restoring, the value of the control resistor 38 is selected suchthat a current less than the required holding current is delivered tothe SCR 26 after it is fired. This will automatically cause the SCR 26to return to its nonconductive state, thereby reinstating normaloperation of the power supply circuit 10. If the overload conditionstill exists, the SCR 26 will, of course, be triggered again and theabove described sequence of operations will be repeated. However, if thecontrol resistor 38 is selected to deliver a current which is equal toor greater than the holding current required by SCR 26, the SCR 26 willremain in its conductive state once an overload condition has beendetected. If designed in this manner, the protective circuit 24 willrender the power supply circuit 10 inoperative until the protectivecircuit is reset, as by manually switching off the DC source 10, forexample.

If the optional capacitor 42 is included in the Darlington circuit 16,it will operate in substantially the same manner as set forth in detailin the aboveidentified Roland et al., patent. As described therein, thecapacitor 42 can be used to supply a brief current pulse to the SCR 26to render it conductive for a brief interval. Termination of the currentpulse then causes the SCR 26 to return to its nonconductive state,assuming that the control resistor 38 is selected to supply a currentsubstantially less than the holding current required to maintain the SCR26 in its conductive state.

Thus the optional capacitor 42 can be used to render the protectivecircuit 24 self-restoring.

From the foregoing disclosure it will be apparent that the circuit ofthe present invention provides a highly reliable, fast acting protectivecircuit for a power supply. It will also be apparent that the circuit ofthe present invention is a voltage responsive circuit which is stable,and highly resistant to the effects of spurious AC voltages.

Obviously numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

Accordingly what is claimed as new and desired to be secured by LettersPatent of the United States is:

l. A voltage responsive protective circuit for a power supply includinga plurality of switching transistors, comprising:

' amplifier circuit means coupled to said switching transistors forsupplying power to said switching transistors,

energy storage means coupled to said switching transistors fordeveloping a variable signal indicative of the operating condition ofsaid switching transistors,

protective circuit means coupled to said energy storage means, saidprotective circuit including electronic switching means selectivelyactuated by said energy storage means in response to the development bysaid energy storage means of a signal indicating a particular operatingcondition of said switching transistors; and,

circuit means coupled between said protective circuit and said amplifiercircuit means for reducing the output of said amplifier circuit means inresponse to actuation of said electronic switching means,

whereby said power supply is protected against damage due to theexistence of said particular operating condition for a prolongedinterval of time.

2. A voltage responsive protective circuit as in claim 1, wherein:

said protective circuit means includes voltage divider means; and,

said voltage divider means is coupled to said energy storage means andto said electronic switching means.

3. A voltage responsive protective circuit as in claim 2, wherein:

said electronic switching means comprises a silicon controlled rectifierincluding a gate electrode; and,

said gate electrode is coupled to said voltage divider means.

4. A voltage responsive protective circuit as in claim 1, wherein:

said amplifier circuit means includes a control electrode,

said circuit means includes a current limiting means for limiting themaximum current flow through saidprotective circuit means; and, saidcircuit means 15 coupled to said control electrode. 5. A voltageresponsive protective circuit as in claim 4, wherein:

1, wherein:

said amplifier circuit means includes current control means coupled tosaid circuit means for returning said electronic switching means fromits actuated state to its inactuated state.

7. A voltage responsive protective circuit as in claim 1, wherein:

said protective circuit means includes second energy storage means forpreventing said electronic switching means from being actuated byspurious signals. g t

8. A voltage responsive protective circuit as in claim 7, wherein:

said electronic switching means comprises a silicon controlled rectifierincluding a gate electrode; and,

said second energy storage means is coupled to said gate electrode.

9. A voltage responsive protective circuit as in claim 1, wherein:

said amplifier circuit means includes current control means coupled tosaid circuit means for preventing said electronic switching means fromreturning to its inactuated state from its actuated state. 10. A voltageresponsive protective circuit, as in claim 1, wherein;

said variable signal indicative of the operating condition of saidswitching transistors is a voltage signal; and,

said particular operating condition is an overloadcondition.

1. A voltage responsive protective circuit for a power supply includinga plurality of switching transistors, comprising: amplifier circuitmeans coupled to said switching transistors for supplying power to saidswitching transistors, energy storage means coupled to said switchingtransistors for developing a variable signal indicative of the operatingcondition Of said switching transistors, protective circuit meanscoupled to said energy storage means, said protective circuit includingelectronic switching means selectively actuated by said energy storagemeans in response to the development by said energy storage means of asignal indicating a particular operating condition of said switchingtransistors; and, circuit means coupled between said protective circuitand said amplifier circuit means for reducing the output of saidamplifier circuit means in response to actuation of said electronicswitching means, whereby said power supply is protected against damagedue to the existence of said particular operating condition for aprolonged interval of time.
 1. A voltage responsive protective circuitfor a power supply including a plurality of switching transistors,comprising: amplifier circuit means coupled to said switchingtransistors for supplying power to said switching transistors, energystorage means coupled to said switching transistors for developing avariable signal indicative of the operating condition Of said switchingtransistors, protective circuit means coupled to said energy storagemeans, said protective circuit including electronic switching meansselectively actuated by said energy storage means in response to thedevelopment by said energy storage means of a signal indicating aparticular operating condition of said switching transistors; and,circuit means coupled between said protective circuit and said amplifiercircuit means for reducing the output of said amplifier circuit means inresponse to actuation of said electronic switching means, whereby saidpower supply is protected against damage due to the existence of saidparticular operating condition for a prolonged interval of time.
 2. Avoltage responsive protective circuit as in claim 1, wherein: saidprotective circuit means includes voltage divider means; and, saidvoltage divider means is coupled to said energy storage means and tosaid electronic switching means.
 3. A voltage responsive protectivecircuit as in claim 2, wherein: said electronic switching meanscomprises a silicon controlled rectifier including a gate electrode;and, said gate electrode is coupled to said voltage divider means.
 4. Avoltage responsive protective circuit as in claim 1, wherein: saidamplifier circuit means includes a control electrode, said circuit meansincludes a current limiting means for limiting the maximum current flowthrough said protective circuit means; and, said circuit means iscoupled to said control electrode.
 5. A voltage responsive protectivecircuit as in claim 4, wherein: said amplifier circuit means includescapacitor means coupled to said control electrode and to said currentlimiting means for delivering a current pulse to said protective circuitmeans.
 6. A voltage responsive protective circuit as in claim 1,wherein: said amplifier circuit means includes current control meanscoupled to said circuit means for returning said electronic switchingmeans from its actuated state to its inactuated state.
 7. A voltageresponsive protective circuit as in claim 1, wherein: said protectivecircuit means includes second energy storage means for preventing saidelectronic switching means from being actuated by spurious signals.
 8. Avoltage responsive protective circuit as in claim 7, wherein: saidelectronic switching means comprises a silicon controlled rectifierincluding a gate electrode; and, said second energy storage means iscoupled to said gate electrode.
 9. A voltage responsive protectivecircuit as in claim 1, wherein: said amplifier circuit means includescurrent control means coupled to said circuit means for preventing saidelectronic switching means from returning to its inactuated state fromits actuated state.